Among modulation/demodulation methods used in optical communication systems, the DPSK (Differential Phase Shift Keying) modulation/demodulation method is a modulation/demodulation method having superior reception sensitivity. Therefore the expectation is that the method will be used particularly in long-distance optical communication systems (see, for example, Non-Patent Document 1). Further, in order to draw the most out of the performance of the receiver for performing DPSK demodulation, it is necessary to provide a delay adjuster and a variable attenuator in the subsequent stage of an interferometer in the receiver as shown in FIG. 3 of Non-Patent Document 2.
FIG. 1 is a block diagram to show the configuration of a typical optical reception device using a DPSK demodulation method. It is noted that an optical reception device having a similar configuration to that of the optical reception device shown in FIG. 1 is shown in FIG. 3 of Non-Patent Document 2.
The optical reception device shown in FIG. 1 includes one-bit delay interferometer 130 for receiving an ultra-high speed optical signal of R bps (bits per second) where R is a number of giga-order, delay adjusters 203 and 204 are for adjusting the delay amount of optical signals 201 and 202, variable attenuators 205 and 206 are for adjusting the amplitude of the optical signals where delay amount is adjusted, optical-electrical signal converter (dual PD (photo-detector)) 209 is for converting optical signals 207 and 208 into an electrical signal, amplifier 211 is for amplifying electrical signal 210, and discriminator 213 is for converting electrical signal 212 into digital data.
One-bit delay interferometer 130 is for example a Mach-Zehnder interferometer. One-bit delay interferometer 130 includes optical branching section 131 for bifurcating the inputted light, transmission paths 132 and 133 for delaying one of the two branched signals with respect to the other signal, and directional coupler 134 for causing the two signals to interfere with each other and to be converted into optical intensity signals.
Delay adjusters 203 and 204 adjust to equalize the phases of the two differential signals, which have been outputted from directional coupler 134 in one-bit delay interferometer 130 before being immediately converted into a single signal. Optical variable attenuators 205 and 206 are used to adjust the signal balance (intensity ratio) between a positive component (logic 1) and a negative component (logic 0) of the signal to be inputted to discriminator 213.    Non-Patent Document 1: Christian Rasmussen et. al., “DWDM40G Transmission Over Trans-Pacific Distance (10000 km) Using CSRZ-DPSK, Enhanced FEC, and All-Raman-Amplified 100-km Ultra Wave Fiber Spans,” Journal of Lightwave Technology, U.S., January 2004, vol. 22) no. 4, pp. 203-207.    Non-Patent Document 2: Jeffrey H. Sinsky, et. al., “A 40-Gb/s Integrated Balanced Optical Front End and RZ-DPSK Performance, IEEE Photonics Technology Letters,” U.S., August 2003, vol. 15, no. 8, pp. 1135-1137.